Robert Furchgott, Louis Ignarro and Ferid Murad have won the 1998 Nobel Prize in Physiology or Medicine for their work on the role played by nitric oxide in transporting signals around the body's cardiovascular system - the heart and the network of blood vessels.

The scientists pieced together the evidence that proved nitric oxide, a gas, was the molecule regulating blood pressure and blood flow.

Viagra boosts the effects of the gas

The pharmaceutical company Pfizer used this discovery to develop its wonder drug Viagra. The blue diamond-shaped pills boost the effects of nitric oxide in a man's body, helping him to achieve erection by dilating the blood vessels in his penis.

The Swedish Karolinska Institute, which judges the prestigious Nobel award, said in its citation: "It was a sensation that the simple, common air pollutant [nitric oxide], which is formed when nitrogen burns...could exert important functions in the organism."

New drugs

The research promises a new generation of drugs to tackle a variety of problems in the cardiovascular system.

Studies of the gas have also shown that white blood cells use it to kill infectious invaders, and that it also can defend against tumours. This has led to tests to see whether nitric oxide could be used to halt tumour growth.

Scientists also believe the gas plays an important role in our sense of smell and memory.

Ferid Murad said on Monday he was surprised, exited and very happy with his award.

"I can not say that I expected it. I have been pondering the odds and thought that maybe I could win the prize, if not now maybe in a couple of years. But I have been trying not to care about it, so it was a pleasant surprise."

Human health

The prize is worth 7.6m kronar ($978,000) and is divided equally among the three scientists.

Winner: Louis Ignarro

Last year, the prize went to Stanley B. Prusiner of the University of California at San Francisco for his discovery of prions, the rogue proteins identified as causing "Mad Cow" Disease.

The winners are not generally known outside the medical community, but their discoveries have far-reaching consequences for human healthcare. Past laureates include Ivan Pavlov, tuberculosis pioneer Robert Koch, and DNA researchers Francis Crick and James Watson.

The medicine prize was the second of the six Nobels to be announced this year. Last Thursday, the literature prize went to Portuguese novelist Jose Saramago.